Bladder-free fuel tank

ABSTRACT

A fuel tank or bladder-free fuel tank having a reservoir on its inside area, limited to an outside area by a double-walled structure. The double-walled structure has an outer wall and an inner wall. A rib structure includes at least one rib extending inwardly into the inside area of the bladder-free fuel tank from the double-walled structure.

This nonprovisional application claims priority under 35 U.S.C. §119(a)to European Patent Application No. 14189525.0, which was filed inGermany on Oct. 20, 2014, and which is herein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an additional fuel tank, particularlyto a bladder-free fuel tank for aircraft applications in which the netvolume to be occupied by fuel is maximized and mechanical stiffness ofthe bladder-free fuel tank is enhanced.

2. Description of the Background Art

U.S. Pat. No. 7,093,470 B2 is related to methods of making integrallystiffened axial load carrying skin panels for primary aircraft structureand fuel tank structures. According to U.S. Pat. No. 7,093,470 B2, amethod of making a skin panel is disclosed according to which a startingplate is machined to a final part geometry including a skin and at leastone integral stiffener machined into the plate and extending generallyoutwardly from the skin. Further, an explosive forming of the machinedplate is performed with its integral stiffener against a forming surfaceof a rigid dye having a contour which at least substantially is inaccordance with the desired curvature for the skin panel, the explosiveforming causing the machined plate to substantially conform to thecontour of the forming surface to at least one of an uniaxial curvatureand a biaxial curvature. Stiffening features are ribs, stiffeners,stringers, frames outer chords and the like.

WO 2007 121969 A1 is related to an inner container surrounded by anouter container used for receiving a cryogenic liquid. Disclosed is aflat inner container, especially an internal tank for a road vehiclewhich is surrounded by an outer container and is used for receiving acryogenic liquid, particularly fuel. The inner container ischaracterized by a combination of the following features: Alongitudinally extending monolithic base with a top wall and a bottomwall, which are connected to also longitudinally extending sidewalls andwith at least two longitudinally extending, substantially straight websthat connect the bottom wall to the top wall so as to form at least onelongitudinally extending chamber. The chamber is arranged between thewebs extending along the entire length of the base as well as from thebottom wall to the top wall and has a predetermined width between thewebs. At least two caps tightly seal the two open ends of the base atthe periphery. The top wall and/or the bottom wall is/are provided withan arch relative to a planar reference top wall and/or reference bottomwall, the distance of the arch between the inner contour of the top walland/or the bottom wall and the planar reference top wall and/or bottomwall amounting to less than 30% of the width of the chamber in thecenter between the webs.

US 2012/01 81 288 A1 is related to a tank having integral restrainingelements and an associated fabrication method. According to US2012/0181288 A1 a tank and an association fabrication method aredisclosed which may limit the damage otherwise occasioned by the impactof a ballistic projectile. The tank may include a wall assembly definedbetween outer and inner walls and a plurality of restraining elementsthat extend between the walls. The restraining elements may be formed tohave a plurality of layers of material that form not only therestraining element but also portions of the inner and/or outer walls.For example, the tank may include a plurality of cells positionedadjacent to one another with each cell forming portions of two adjacentrestraining elements and portions of the inner and/or outer walls. Acorresponding method for fabricating a tank including a wall assemblyhaving a plurality of restraining elements is also provided.

U.S. Pat. No. 7,090,167 B2 is related to a method and apparatus forliquid containment such as for aircraft fuel vessels. A vessel caninclude a first surface portion, a second surface portion spaced apartfrom the first surface portion and a core positioned between the firstand second surface portions. The core can be sealably connected to thefirst and second surface portions and can be positioned to carry a loadfrom at least of the first and second surface portions to the other. Thecore can include a plurality of cells separated by cell walls, at leastsome of which have wall openings positioned to provide liquidcommunication between adjacent cells.

U.S. Pat. No. 8,550,403 B2 is related to an aircraft fuel tank. Anaircraft fuel tank is capable of suppressing electrostatic charging offor example by flow electrification with the fuel. The aircraft fueltank comprises an upper skin and a lower skin that exhibit conductivityand form a portion of a container for storing fuel, an internalstructure formed from metal and an inner surface layer which hassemiconductor properties or insulating properties and is formed in anintegral manner on the inner surfaces of the upper skin and the lowerskin in the location where the internal structure contacts the upperskin and the lower skin, as well as in the surrounding portion thereof,wherein the inner surface layer is formed at least in the surroundportion from a material having semiconductor properties.

EP 2 048 079 A2is related to a fuel tank assembly in an associatedmethod. A fuel tank assembly and associated method are provided whichmay limit the damage otherwise occasioned by the impact of a ballisticprojectile. The fuel tank assembly may include a bladder defined betweeninterior and exterior bladder walls, with the interior bladder walldefining a volume for storing fuel. The exterior bladder wall is atleast as more rigid than the interior bladder wall. The fuel tankassembly can also include connectors between the exterior and interiorbladder walls to at least partially limit the expansion of the bladder.The fuel tank also includes an inlet opening to the bladder to permit apressurized gas to be introduced into the bladder. The fuel tankassembly may also include a valve through the interior bladder wall intothe volume for storing fuel to permit at least some of the pressurizedgas to be introduced therein.

U.S. Pat. No. 5,320,247 is related to storage tanks with internalsupport ribs. A method of forming a storage tank from an existing matrixtank comprises adding spaced internal support ribs. The interior surfaceof the matrix tank is covered with a layer of fibrous reinforcedresinous material to form a main body and internal support ribs areadded. The main body and support ribs are secured together such that theribs protrude inwardly. The formed tank is fully and independentlycapable of containing liquid and withstanding normal internal andexternal load forces. A double walled formed storage tank system is alsoprovided wherein an inner wall between or on the internal support ribsis added.

Fuel tanks and/or additional fuel tanks such as additional center tanksfor aircraft applications having bladders integrated therein aremanufactured very costly and constitute a compromise in view of thefactors weight, production and maintenance costs and in view ofintegration of in-fuel-tank equipment. Further, the use of a bladder ina fuel tank comes along with a loss of usable tank volume. Knownhoneycomb-structured tanks suffer the drawback of being applicable onlysubject to specific design, production and maintenance know-how tocreate an additional fuel tank or a fuel tank for an aircraftapplication with acceptable costs and economical risk.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide for a fueltank, particularly a bladder-free fuel tank having an enhanced rigidity.

A further object of the present invention is to provide for abladder-free fuel tank, having an enhanced fuel storage capacity.

A still further object of the present invention is to provide for a fueltank having a reduced weight, particularly for aircraft applications.

According to an exemplary embodiment of the present invention a fueltank, particularly a bladder-free fuel tank is provided, having areservoir on its inside area limited to an outside area by adouble-walled structure wherein the double-walled structure comprises anouter wall and an inner wall, a rib structure of a plurality of ribsextending inwardly into the inside area from the double-walledstructure.

The bladder-free fuel tank according to the present invention providesfor a storage capacity for a maximum of fuel volume since no bladder ispresent in the inside area of the bladder-free fuel tank according tothe present invention. Still further, the rigidity of the bladder-freefuel tank according to the present invention is enhanced significantlysince the inwardly extending rib structure provided within the interiorof the bladder-free fuel tank increases the mechanical rigidity thereofsignificantly. Due to the lack of a bladder the double-walled structure,limiting the interior of the tank of the surrounding, can be placedcloser to the outside of the bladder-free fuel tank, i.e. closer to oralmost on the edge of a ground-plate having e.g. a rectangular or asquared shape. Thus, the storage volume of fuel within the bladder-freefuel tank according to present invention is maximized.

According to an embodiment the present invention, the double-walledstructure comprises an outer wall or outer wall segments beingmanufactured from a material including aluminum and the outer wall orouter wall segments being manufactured as a sheet metal part. Stillfurther, the double-walled structure of the bladder-free fuel tankaccording to the present invention comprises an inner wall and/or innerwall segments which are machined or milled components being made from amaterial, aluminum for example.

The components of the double-walled structure can be fixed to oneanother by means of a spacer which on the one hand defines a width ofthe hollow interior between the components of the double-walledstructure and on the other hand is arranged in mounting areas forfixture elements having a sealing means assigned thereto such as screws,bolts or other kinds of fixture elements. By means of the fixtureelements the components, i.e. the inner wall, the outer wall, inner wallsegments and/or outer wall segments are fixed to one another providing amechanical rigid compound able to withstand tension-, pressure-,bending- and shear-stresses generated during operation of thebladder-free fuel tank according to the present invention. The width ofthe hollow interior of the double-walled structure can be between 3 mmto 10 mm, which is sufficient for drain and venting purposes of thebladder-free fuel.

The inwardly extending rib structure constitutes a reinforcing structurewhich either can be manufactured as separate components, which are to befastened to the inner wall or to inner wall segments of thedouble-walled structure by fixture elements. On the other hand, theinwardly extending rib structure can be manufactured in a one pieceembodiment, i.e. the inner wall or inner wall segments and the inwardlyextending rib or the inwardly extending rib structure is a one-piececomponent, thereby significantly enhancing mechanical rigidity of thebladder-free fuel tank according to the present invention.

In an embodiment, a rib-structure pattern of the inwardly extendingribs, i.e. into the interior of the bladder-free fuel tank, may beshaped as a cross-link structure, diagonal-link pattern structure or aparallel-link pattern structure. The inwardly extending rib structurebeing present in the interior of the fuel tank according to the presentinvention is linked to a ground plate of the bladder-free fuel tank bymechanical elements. By means of inner tied rods for example,neighboured wall structure sections or in particular oppositely arrangedsections of the double-walled structure are connected, thus enhancingsignificantly mechanical rigidity of the bladder-free fuel tankaccording to the present invention. Still further, the plurality ofinwardly extending ribs may comprise buckling resistant T-shaped orL-shaped extensions which are fastened on the surface of the groundplate of the bladder-free fuel tank according to the present invention.

The double-walled structure can be arranged without interruptions alongthe entire circumference of the ground plate of the bladder-free fueltank according to the present invention. In overlapping regions withrespect to the outer wall and the inner wall of the double-walledstructure of the bladder-free fuel tank, edge portions of outer wallsegments overlap each other and are sealingly connected via fixturessuch as screws or bolts or the like. Alternatively, within overlappingregions of outer wall segments a sealing connection between the outerwall segments is obtained by welding such as automatic laser welding.Also, within overlapping regions of inner walls segments a sealingconnection between the inner wall segments is obtained by welding suchas automatic laser welding to give an example. In a further advantageousembodiment of the bladder-free fuel tank according to the presentinvention, edge portions of inner wall segments within an overlappingregion are embedded in recesses of corresponding inner wall segments sothat upon interconnection of the edge portion in the presence of asealing means a substantially flat outer surface of the inner wall orinner wall segments towards the hollow interior of the double-walledstructure is obtained.

The bladder-free fuel tank according to an embodiment of the presentinvention may contain local strengthenings or machine-milled components.Instead of the use of spacers to define the width of the hollow space ofthe double-walled structure in the outer wall or inner wall or outerwall segments and/or inner wall segments dome-shaped elements can bemanufactured therein upon production thereof, thus defining a width ofthe hollow space of the double-walled structure as well as constitutinga mounting area of the fixture elements such as screws and bolts or thelike.

In an embodiment of the bladder-free fuel tank according to the presentinvention, the inwardly extending rib structure can be manufactured in aheight which is close to the height of the components e.g. slosh wallsof the double-walled structure to prevent excessive sloshing of fuelwithin the interior of the bladder-free fuel tank according to thepresent invention.

Still further, the present invention relates to the use of thebladder-free fuel tank as additional center tank for aircraftapplications.

The bladder-free fuel tank according to an embodiment of the presentinvention as will be described in further detail herein below comesalong with a number of significant advantages. It deserves mentioningthat the bladder-free fuel tank according to the present invention isbladder-free so that consequently the weight of the bladder is no longera significant fact. Since the bladder structure does not contribute tothe mechanical strengthening of the additional center tank, the bladderhas the disadvantage to contribute to additional weight only. By meansof the present invention this major drawback is eliminated.

The cost for a bladder can be saved as well as mounting and maintenancecosts. Since the interior of the bladder-free fuel tank according to thepresent invention is no longer occupied by a bladder, and in particularsince the stiffening ribs are extended inwardly which allows tosignificantly increase the wettable tank volume, the usable capacity ofthe interior of the bladder-free fuel tank according to the presentinvention is significantly optimized. Still further, the double-walledstructure of the bladder-free fuel tank according to the presentinvention is suitable for reliable production and mounting methods sincethe components such as sheet metal parts and milled components arereliable to manufacture. In case the components are made of light-weightmaterials such as aluminum, however, providing for a significantmechanical rigidity, such materials are easy to manufacture in view ofthe portion of loss of material during production, forming and milling,respectively.

The double-walled structure is relatively easy to maintain, the surfaceprotection versus corrosion is safe, the bladder-free fuel tankaccording to the present invention can very easily be connected into analready existent fuel supply system. Since no bladder is present in thebladder-free fuel tank, in-fuel-tank equipment is relatively easy tomount—e.g. directly to mechanical interfaces machined into the innerwall and without to require a separate bracket or fixture. When comparedto solutions according to the prior art the number of openings in thedouble-walled structure for maintenance purposes or for e.g. integrationgauge probes is minimized. In the interior of the bladder-free fuel tankaccording to the present invention, the inwardly extending ribsconnected to the inner wall or inner wall segments of the double-walledstructure are connected to each other by additional riveted ribs or byone or several tie rods, which may be arranged in the interior of thebladder-free fuel tank alternatingly. The weight of the bladder-freefuel tank according to the present invention is minimized by means ofthe high stress load resistance double-walled structure and furthermorewhen manufactured from materials including aluminum or magnesium. Therigidity of the bladder-free fuel tank according to the presentinvention is enhanced in case the inwardly extending ribs or a relevantnumber of them are made in a sufficient height in order to preventexcessive sloshing of fuel. The inwardly extending ribs may additionallyserve as anchoring points for inner tie rods or compression struds, ormay serve as connecting points of separate slosh walls which also mayact as functionally integrated tie-rods.

The ground plate of the bladder-free fuel tank according to the presentinvention is no longer covered by the bladder which does not contributeto mechanical stiffness and causes weight only. The bladder-free fueltank according to the present invention has in its interior inwardlyextending ribs, thus enhancing rigidity while an outer space envelope isnot influenced by the ribs now integrated into the interior of thebladder-free fuel tank according to the present invention. Since theribs are now present in the interior of the tank, the surrounding of thebladder-free fuel tank according to the present invention is free ofreinforcing structures such as ribs. According to the present inventiona shifting of the double-walled structure to a maximum space envelope isachieved so that the storage capacity of the bladder-free fuel tankaccording to the present invention is enhanced significantly whencompared with fuel tanks using a bladder. By means of the presentinvention a significantly improved usable fuel to tank structure ratiocan be obtained as well as a significantly improved ratio between usablefuel to tank outline volume when compared with prior art solution. Stillfurther, the ratio between the usable fuel to the tank manufacturingcost can be significantly improved by the use of the present invention.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus, are not limitiveof the present invention, and wherein:

FIG. 1 shows a cross section through a double-walled structure having anouter wall and an inner wall with inwardly extending reinforcing ribs;

FIG. 1.1 shows an embodiment of a fixture and a dome present on theinner wall or the inner wall segment;

FIG. 1.2 shows an embodiment of a fixture element, connecting an innerwall and an outer wall of the double-walled structure;

FIG. 2 shows an embodiment of a rib pattern present in the interior ofthe bladder-free fuel tank according to the present invention;

FIGS. 2a and 2b show details of milled pockets machined in the inwardlyextending ribs;

FIG. 3 shows a cross-section of the double-walled structure withoverlapping regions present in the inner wall and the outer wall;

FIG. 4 shows the double-walled structure with fixture elements mountedin mounting areas of the fixtures and the resulting and a formed outerwall of the double-walled structure of the bladder-free fuel tank;

FIG. 5 shows a top view of a large additional riveted rib and a part ofa tie rod inside the bladder-free fuel tank;

FIG. 6 shows a side view of a fuel tank according to the presentinvention;

FIG. 7 shows an embodiment of a plurality of force transmission pointsarranged between the double-walled structure and a part of an inner tierod;

FIG. 8 shows a rib pattern being arranged substantially triangular;

FIG. 9 shows a parallel rib pattern, the reinforcing ribs having milledpockets;

FIG. 10 shows a substantially parallel rib pattern, the ribs arrangedwith respect to the respective fixtures in an alternate fashion;

FIG. 11 shows an arch-shaped section of a double-walled structure in aradius with respect to a first axis A; and

FIG. 12 shows a corner design embodiment of the double-walled structurewith an internally arranged fixture element for inner wall segments andan externally arranged fixture element within the outer envelopeconnecting outer wall segments.

DETAILED DESCRIPTION

FIG. 1 shows a cross section through a double-walled structure of thebladder-free fuel tank according to the present invention.

A bladder-free -fuel tank 10 has an inside area 12 which is limited froman outside area 14, i.e. the surrounding, by a double-walled structure16. The double-walled structure 16 may comprise an outer wall 18 and aninner wall 20 or may comprise—as best shown in FIG. 3—a first inner wallsegment 72, a second inner wall segment 74 interconnected to each other.The outer wall 18 may comprise a first outer wall segment 78 and asecond outer wall segment 80, likewise best shown in FIG. 3.

As can be derived from the cross-section according to FIG. 1, thedouble-walled structure 16 of the bladder-free fuel tank 10 according tothe present invention comprises a number of fixture elements 22 such asbolts 38 or screws 42, respectively, by means of which the outer wall 18and the inner wall 20 are fixed to one another. The double-walledstructure 16 according to FIG. 1 comprises a hollow space 26 establishedbetween the outer wall 18 and the inner wall 20. The hollow space 26,the width 27 of which is defined by a spacer 28 or in the alternative bydome-shaped elements 34 present the inner wall 20, serves as a drain- orvent-channel of the bladder-free fuel tank 10 according to the presentinvention. Each fixture element 22 being mounted in a mounting region ofthe double-walled structure 16 may comprise a sealing 30, a sealing ring32 for instance.

As can be derived from the cross-section according to FIG. 1, inwardlyextending ribs 36 are present, which extend from the inner wall 20 ofthe double-walled structure 16 in direction to the inside area 12 of thebladder-free fuel tank 10 according to the present invention. Theinwardly extending ribs 36, which form a rib structure, a rib pattern56, see FIG. 2, may be part of a machine-milled inner wall 20 or ofmachine-milled inner wall segments 72, 74, respectively, as best shownin FIG. 3. In the alternative, the inwardly extending ribs 36 shown inFIG. 1 extend into the inside area 12, i.e. the interior of thebladder-free fuel tank 10 according to the present invention enhancingrigidity and mechanical stiffness of the bladder-free fuel tank 10.Instead of a one-part component, i.e. the inner wall 20 and the inwardlyextending ribs 36 being a one-part arrangement, the inwardly extendingribs 36 may constitute separate components which are fastened to theinner wall 20 or to machine-milled inner wall segments 72, 74,respectively, of the double-walled structure 16.

In the mounting areas, i.e. in the region where the fixture elements 22,i.e. screws 42 or bolts 38 are present, a threaded connection 40 isestablished between the outer wall 18 and the inner wall 20. Since theinner wall 20 or respective inner wall segments 72, 74 aremachine-milled, the respective threadings for establishing a threadedconnection 40 can be milled or drilled upon production of the respectiveinner wall 20 or the respective inner wall segment 72, 74 respectively,as best shown in FIG. 3.

As shown in FIG. 1, the double-walled structure 16 is a compoundcomprising an inwardly arranged inner wall 20 being machine-milled, i.e.a milled component and an outwardly arranged sheet metal part. Bothcomponents, i.e. the outer wall 18 and the inner wall 20, are made froma material including aluminum. In the embodiment given in FIG. 1 theinner wall 20 and the outer wall 18 are spaced from one another byspacers 28 establishing a width 27 of the hollow space 26 of thedouble-walled structure 16.

Both walls, i.e. the outer wall 18 and the inner wall 20 of thedouble-walled structure 16, are fastened to one another in order toendure tension, pressure, bending and shear forces by being graduallydeformable, providing a high mechanical rigidity. The drain and ventchannel 24 within the double-wall structure 16 has a width 27 of a fewmillimeters only.

FIG. 1.1 shows a fixture element of the double-walled structure ingreater detail.

According to the exemplary embodiment given in FIG. 1.1 the inner wall20 comprises a dome element 34 serving as support for the threadedconnection 40. As can be derived from FIG. 1.1 the height of the domeelement 34 defines the width 27 of the hollow space 26 between the outerwall 18 and the inner wall 20 of the double-walled structure 16 of thebladder-free fuel tank 10. As can be derived from FIG. 1.1 the fixtureelement 22 is a screw having a washer-type sealing ring 32 performing asealing function. In inward direction, i.e. in direction of the insidearea 12 of the bladder-free fuel tank 10 according to the presentinvention, the inwardly extending rib 36 is shown having both sidesmilled.

FIG. 1.2 shows an alternative embodiment of a fixture element of thedouble-walled structure.

As can be derived from FIG. 1.2 the fixture element 22 is rather a bolt38 than a screw 42 as shown in FIG. 1.1. In the mounting area of thedouble-walled structure 16 the bolt 38 fixes the outer wall 18 to therespective inner wall 20 provided with dome-shaped elements 34 definingthe width 27 of the hollow space 26 of the double-walled structure 16.As can be derived from FIG. 1.2 in this embodiment, the inner wall 20 ofthe double-walled structure 16 and the inwardly extending rib 36constitute a one-piece embodiment. The sealing 30 according to theembodiment given in FIG. 1.2 is achieved by means of a washer-typesealing ring 32.

FIG. 2 shows a rib pattern present in the inside area of thebladder-free fuel tank 10 according to the present invention.

The cross-section given in FIG. 2 shows a section of a rib pattern 56within which the inwardly extending ribs 36 are arranged in arectangular pattern. Each of the inwardly extending ribs 36 isorientated in a 90°-degree orientation within the rib pattern 56 asgiven in FIG. 2. The inwardly extending ribs 36 of the rib pattern mayhave a first length 50 and a second length 52 which may differ from oneanother or may be equal with respect to one another. A ratio between thefirst distance 50 and the second distance 52 as shown with respect tothe length of the inwardly extending ribs 36 may vary.

In FIG. 2, the rib pattern 56 nearly is a square-shaped pattern,however, different patterns for instance x-cross pattern, diagonal ribpattern 58 or parallel rib pattern 60 may be feasible as well. In localthickenings 48 quantities of additional material are provided to reduceany stress level during operational material deflection below acrack-propagation level, resulting in an effective crack stop design.The rib pattern 56 given in FIG. 2 is arranged such that a crossing oftwo ribs 56 corresponds to fixture elements 22. In the top view given inFIG. 2, a section of the interior of the inside area 12 of thebladder-free fuel tank 10 according to the present invention is shown.The inwardly extending ribs 36 may comprise buckling resistant T-shapedextensions preventing expansions 86.

FIGS. 2a and 2b , respectively, show details of machine-milled pocketsprovided in the inwardly extending ribs 36.

According to the details given in FIGS. 2a and 2b , respectively, theinwardly extending ribs 36 are provided with milled pockets 130. Themilled pockets 130 form openings in the inwardly extending ribs 36 toallow for the flow of fuel between different compartments arranged inthe interior of the bladder-free fuel tank 10 according to the presentinvention. By means of the milled pockets 130 a uniform, homogeneousfuel level can be established within all compartments within theinterior of the bladder-free fuel tank 10 according to the presentinvention.

FIG. 3 shows a cross-section through a double-walled structure 16 in theregion of a 90°-degree bow 76.

According to FIG. 3, the double-walled structure 16 of the bladder-freefuel tank 10 according to the present invention comprises overlappingregions 70. To prevent mechanical stress, the overlapping regions 70 arearranged not directly within a 90°-degree bow 76 but rather in straightportion of the double-walled structure 16. As can be derived from thetop view given in FIG. 3, the double-walled structure 16 with respect tothe outer wall 18 and the inner wall 20, respectively, comprise theoverlapping regions 70 within which either inner wall segments 72, 74contact one another and/or outer wall segments 78, 80, respectively, areinterconnected with one another. Within the overlapping regions 70respective edge portions 82, 84 of the first inner wall segment 72 andthe second inner wall segment 74 are interconnected with one another bymeans of a fixture element 22. The overlapping regions 70 may comprisethe first outer wall segment 78 and the second outer wall segment 80overlapping each other or as likewise shown in FIG. 3 may comprise anoverlapping region 70 within which one edge portion 82 of a first innerwall segment 72 is embedded in a recess 88 of a corresponding edgeportion 84 of the second inner wall segment 74, respectively. In thesecond alternative, a flat surface of the inner wall 20 towards thehollow space 26 present within the double-wall structure 16 is achieved.

Within the overlapping regions 70 the edge portions 82, 84,respectively, may be interconnected either via fixture elements 22 suchas bolts 38 or screws 42 having a sealing 30 such as a sealing ring 32.In the alternative, within the overlapping regions 70 the edge portions82, 84, respectively, may be sealingly interconnected with each other bymeans of automatically performed laser welding or the like.

Within the overlapping regions 70 likewise edge portions of outer wallsegments 78, 80, respectively, may be interconnected either via fixtureelements 22 such as bolts 38 or screws 42 having a sealing 30 such as awasher-type sealing ring 32. In the alternative, within the overlappingregions 70 the edge portions may be sealingly connected with each otherby means of automatically performed laser welding or the like.

Within the 90°-degree-bow 76 according to FIG. 3, a local strengtheningof the milled component is achieved. As can be seen in the cross-sectionview given in FIG. 3, the inwardly extending ribs 36 may comprisebroader buckling resistant T-shaped or L-shaped extensions 86 fastenedor not fastened to a ground plate 44 onto which the double-walledstructure 16 of the bladder-free fuel tank 10 sealingly is mounted.

According to an exemplary embodiment of the present invention the groundplate 44 is sealingly fixed to the double-walled structure 16 of thebladder-free fuel tank 10 according to the present invention. Thus, acontinuous interruption-free circumferentially extending double-walledstructure 16 is established thereby maximizing the capacity of thereservoir in the interior of the bladder-free fuel tank 10 according tothe present invention. Still further—however not shown in theembodiments given in FIG. 3—inner tie rods 106 or compression struds 110may be present, preferably arranged between neighbouring or oppositelyarranged sections 112 of the double-walled structure 16. By means of theinner tie rods 106 an inner pressure imposing a local mechanical load onthe double-walled structure 16 is reduced significantly. The inner tierods 106 may be connected to the inwardly extending ribs 36 by means ofbolts 38 or screws 42. Plate-shaped connectors 96 as best shown inconnection with FIG. 7 are arranged between the inwardly extending ribs36 and the inner tie rods 106 (shown in FIGS. 5 and 7), thus allowingfor a connection of oppositely arranged sections 112 of thedouble-walled structure 16 providing a plurality of force transmissionpoints 108, as best shown in FIGS. 5 and 7, respectively.

The inwardly extending ribs 36 as shown in FIGS. 1, 2 and 3 are made ina height 90, 92 corresponding to the height of the double-walledstructure 16, and/or large additional e.g. riveted ribs 102 as bestshown in FIG. 6 and/or tie rods made in a height, so that the insidearea 12 of the bladder-free fuel tank 10 according to the presentinvention as precautionary measure is protected against excessivesloshing of fuel. According to the present invention the fixtureelements 22 may be realized by the bolts 38 or the screws 42establishing the threaded connection 40 between the outer wall 18 andthe inner wall 20 of the double-walled structure 16. Alternatively, thefixture elements 22 may be realized as shear-sleeves including athreaded connection. By means of local deepened drawing of the sheetmetal part, i.e. the outer wall 18, a forming of a flat-shaped deepeningis performed, followed by a subsequent connection with the shear sleevesand/or a spacer such as a washer.

FIG. 4 shows a top view of a cross-section through the double-walledstructure 16, fixture elements 22 mounted in mounting areas 46 thereof.As can be derived from embodiment given in FIG. 4, the outer wall 18 ofthe double-walled structure 16 is bulged by arrangements of fixtureelements 22 in the region of the respective mounting areas 46 againstthe inner wall 20 of the respective double-walled structure 16. Thuswithin the bulging regions 100 of the outer wall 18 the hollow space 26defining the drain- or vent channel 24 is locally interrupted.

In the embodiment given in FIG. 4 the fixture elements 22 are screws 42which establish a threaded connection 40 to respective threadingsarranged in local thickenings 48 of the inner wall 20. In the embodimentgiven in FIG. 4 the inner wall 20 of the double-walled structure 16 andthe inwardly extending ribs 36 are a one-part-embodiment. The inwardlyextending ribs 36 may have one milled side or may have both sidesmilled.

The sealing of the double-walled structure 16 is achieved by themachine-milled inner wall 20, which in the overlapping regions 70 asbest shown in FIG. 3 has a sealing 30 such as sealing paste or the like.Still further, the sealing 30 of the inside area 12 of the bladder-freefuel tank 10 according to the present invention is established byinterconnected edge portions 82, 84, respectively, of first and secondinner wall segments 72, 74, respectively. A sealing 30 between theground plate 44 and the double-walled structure 16 of the bladder-freefuel tank 10 is likewise realized by circumferentially extending sealingmeans. The connection of the ground plate 44 and the double-walledstructure 16 of the bladder-free fuel tank 10 according to the presentinvention is performed likewise to the connection between according toFIG. 3 or 13, respectively.

The bladder-free fuel tank 10 according to the present inventioncomprises a rigid connection of the outer wall 18 or outer wall segments78, 80, respectively with the inner wall 20 or inner wall segments 72,74 forming a rigid shear resistant structure which is subject topressure differences between i.e. the inside area 12 and the outsidearea 14 of the bladder-free fuel tank 10. By means of the inwardlyextending reinforcing ribs 36 and the inner tie rods 106 or compressionstruds 110, respectively, the bladder-free fuel tank 10 according to thepresent invention has a lower weight as compared to fuel tankarrangements having a bladder arranged within the interior.

The double-walled structure 16 as given in FIGS. 1, 3 and 4,respectively, may comprise openings for fittings allowing fordouble-walled openings for piping and electrical connections or openingsfor maintenance personnel for reasons of inspection. Still further, inthe double-walled structure 16 openings may be present allowing for theinstallation of drain valves or the like. Still further, in thedouble-walled structure 16 elements may be integrated for the connectionof fuel valves and the like without outer parts arranged within theouter space envelope of the bladder-free fuel tank 10. Still further,in-fuel-tank equipment such as valves, piping, sensors, pumps isarranged and can be fastened directly to the inwardly extending ribs 36or the rib pattern arranged in the inside area 12 of the bladder-freetank 10.

FIG. 5 shows a top view of the double-walled structure 16 with assignedthereto large additional riveted ribs and a combination of largeadditional riveted ribs with tie rods connected thereto.

The cross-section according to FIG. 5 shows a double-walled structure16, comprising the outer wall 18 and the inner wall 20. Within mountingarea 46 fixtures elements 22, such as screws 42 or bolts 38 areconnecting the outer wall 18 and the inner wall 20. As can be derivedfrom the cross-section according to FIG. 5, the outer wall 18 has asomewhat bulged or buckled shape, limiting the drain- or vent-channel 24within the hollow space 26 of the double-walled structure 16. Thefixture elements 22, such as screws or bolts, are fixed to respectivethreadings, i.e. constituting a threaded connection 40. Thecorresponding threading is manufactured within local thickenings 48 ofthe inner wall 20. From the inner side of the inner wall 20 of thedouble-walled structure 16 the inwardly extending, reinforcing ribs 36extend in direction to the inside area 12 of the bladder-free fuel tank10 according to the present invention. As can be derived from FIG. 5,additional riveted ribs 102 are fastened to the inwardly extending ribs36 extending into inward direction in the inside area 12 of thebladder-free fuel tank 10. The large additional riveted ribs 102 areconnected via a rivet connection 104 for example, to an upright or to ahorizontally extending portion of the inwardly extending ribs 36.

Still further, by means of a further riveted connection 104, a part oran entire tie rod 106 can be connected to the inwardly extending rib 36.The tie rod 106, a portion thereof given in FIG. 5, extends to anoppositely arranged double-walled structure 16 of the respectivebladder-free fuel tank 10 as schematically shown in FIG. 6.

As can be seen in FIG. 5, e.g. every third of the inwardly extendingreinforcing ribs is provided with a large additional rib 102 connectedvia a rivet connection 104.

FIG. 6 shows a bladder-free fuel tank 10 according to the presentinvention not to scale with an example of an inner tie rod.

As can be derived from this schematic view according to FIG. 6, twooppositely arranged double-walled structures 16 comprising inner walls20 or first and second inner walls 72, 74, respectively. To each of theinner walls 20 a respective large additional e.g. riveted rib 102 isassigned. Via e.g. riveted connections 104 a tie rod 106 is fastened toboth of the oppositely arranged large additional e.g. riveted ribs 102at several force transmission points 108, which may be embodied likewiseas e.g. riveted connections 104 as sketched in the schematic viewaccording to FIG. 6. The inner tie rod 106 enhances mechanical rigidityof the bladder-free fuel tank 10 according to the present invention.

The fuel contained in the interior of the bladder-free fuel tank 10 isconveyed to a respective main tank of a fuel supply of an aircraft bymeans of a pressure difference. The pressure difference may result fromthe pressure difference between the outside air pressure of an aircraftwhen operated at flight level about 10 km and the cabin air pressureamounting to approximately 0.75 bar to 0.9 bar. By means of thispressure difference Δp the fuel contained in the interior i.e. in theinside area 12 of the bladder-free fuel tank 10 is conveyed to the mainfuel supply system for the engines of the respective aircraft. Thus, theinterior, i.e. the inside area 12 of the bladder-free fuel tank 10according to the present invention is pressure-loaded by the pressureprovided e.g. by the cabin air and has to be designed such that thepressure for conveying the fuel present in the bladder-free fuel tank 10according to the present invention imposes a load on the double-walledstructure 16 of the bladder-free fuel tank 10. By means of the at leastone tie rod 106 connecting opposingly arranged sections 112 of thedouble-walled structure 16, the mechanical rigidity of the bladder-freefuel tank 10 according to the present invention is enhancedsignificantly.

The tie rod 106 connecting the large additional riveted ribs 102 to oneanother may be made from sheet metal or another material. The fuelcapacity of the bladder-free fuel tank 10 according to the presentinvention is typically at least ca. 1500 l and may of course containlarger volumes of aircraft fuel.

FIG. 7 shows a schematic view of an inner tie rod connected to aplate-shaped connector which internally is connected to inwardlyextending ribs of the double-walled structure.

As can be derived from the schematic view given in FIG. 7, the inwardlyextending reinforcing ribs 36 which are a part of the inner wall 20 ofthe double-walled structure 16, extend into the interior, i.e. theinside area 12 of the bladder-free fuel tank 10 according to the presentinvention. The inwardly extending reinforcing ribs 36 may have the samelengths or may have different lengths as shown in FIG. 7.

As schematically given in FIG. 7, a substantially plate-shaped connector96 is connected to each of the inwardly extending reinforcing ribs 36 ata number of force transmission points 108. On the other hand, theplate-shaped connector 96 according to FIG. 7 is fastened at a furtherforce transmission point 108 to a part of an inner tie rod 106 whichinternally is fastened to an oppositely arranged plate-shaped connector96 fastened to an opposite section of the respective double-walledstructure 16, however not shown in FIG. 7, of the bladder-free fuel tank10 according to the present invention. The plate-shaped connector 96 maylikewise be a sheet metal component made of aluminum or aluminum alloy,similar to the tie rod 106.

As an alternative to the tie rod arrangements shown in FIGS. 5, 6 and 7,respectively, at the interior, i.e. the inside area 12 of thebladder-free fuel tank 10 according to the present invention, maycomprise compression struds 110 extending between sections of thedouble-walled structure 16 orientated opposite to one another.

FIG. 8 shows a rib pattern of inwardly extending ribs, the rib patternhaving a substantially triangular shape.

According to FIG. 8 a first the ribs 36 in the interior, i.e. the insidearea 12 of the bladder-free fuel tank 10, are spaced from one anotherhorizontally by the first distance 50. Fixture elements 22 are spacedfrom one another by the second distance 52. The first distance 50 andthe second distance 52 may be equal or may vary with respect to oneanother. The triangular rib pattern 62 according to FIG. 8 is furthercharacterized by an angle 64 (α) which is chosen between 30° and 60°particularly 45° but may as well vary, e.g. up to 90° or up to 120°. Indashed-dotted lines, milled pockets 130 are shown. The milled pocketsare milled portions at the bottom of the ribs, perpendicular to thedrawing plane. The milled pockets 130 are arranged close to the groundplate 44 and to the top plate of the bladder-free fuel tank 10 accordingto the present invention to allow for a flow of fuel and air between thecompartments—in this case triangular shaped compartments—limited by thereinforcing ribs 36 in the inside area 12 of the bladder-free fuel tank10. By means of the milled pockets, an exchange of fuel and air betweenneighboring compartments in the inside area 12 of the bladder-free fueltank 10 according to the present invention is feasible.

FIG. 9 shows a part of a parallel rib pattern.

As can be derived from the schematical view according to FIG. 9, theinwardly extending ribs 36 are arranged in a parallel rib pattern 60inside the interior, i.e. the inside area 12 of the bladder-free fueltank 10 according to the present invention. As can be derived from FIG.9 the inwardly extending ribs 36 likewise are equipped with milledpockets 130 connecting neighboring compartments, limited by the inwardlyextending ribs 36 to allow for an exchange of fuel or air betweenneighboring compartments in the inside area 12 of the bladder-free fueltank 10. The fixture elements 22 are spaced apart from one another bythe first distance 50. The parallel rib pattern 60 is furthercharacterized by the second distance 52 between each of the in parallelorientated inwardly extending ribs 36.

FIG. 10 shows a parallel rib pattern similar to the embodiment given inFIG. 9, however, the fixture element being alternately arranged withrespect to neighboring, in parallel orientated inwardly extending ribs.

As is shown in FIG. 10, each inwardly extending rib 36 comprises fixtureelements 22 spaced with respect to one another by the first distance 50.Similar to the embodiment of the parallel rib pattern 60 given in FIG.9, the in parallel orientated inwardly extending ribs 36 are spaced fromone another by the second distance 52.

Contrary to the embodiment given in FIG. 9, fixture elements 22 mayalternate with respect to one another by the alternate fixture pointsdesign 66 as schematically shown in FIG. 10. However, similar to theembodiments given in FIG. 9, the parallel rib pattern 60 according tothe embodiment given in FIG. 10 comprises inwardly extending ribs 36,each having a number of milled pockets 130 arranged at the bottom of theinwardly extending ribs 36. By means of the milled pockets 130 arrangedat the bottom of each of the inwardly extending ribs 36 an exchange offuel or air between neighboring compartments separated by the inwardlyextending ribs 36 from one another is feasible. The milled pockets 130allow for a collection of fuel to be conveyed into a main tank of anarea, the collection being in an integrated sump or a deepening 132 inthe bottom of the bladder-free fuel tank 10, i.e. the integratedsump/deepening 132 being manufactured in the ground plate 44 of thebladder-free fuel tank 10 according to the present invention (see FIG.6).

FIG. 11 shows an arch-shaped section of the double-walled structure 16of the bladder-free fuel tank 10 according to the present invention.

As can be derived from FIG. 11, the double-walled structure 16 may havean arch-shaped section given in part in FIG. 11. The arch-shaped sectionof the double-walled structure 16 according to FIG. 11 is manufacturedin a radius 114 about a first axis 116. Perpendicular to the drawingplane according to FIG. 11, the arch-shaped section of the double-walledstructure 16 as shown in FIG. 11 may additionally be arch-shaped withrespect to a second axis 118, i.e. the bladder-free fuel tank 10according to the present invention in this embodiment having a sphericalor semi-spherical shape, for instance when arranged within a tailsection of an aircraft.

The arch-shaped section of the double-walled structure 16 according tothe embodiment in FIG. 11 comprises the outer wall 18 and the inner wall20 defining the hollow space 26. The width 27 of the hollow space 26 isdefined by the height of dome-shaped elements 34 of the inner wall 20 orinner wall segments 72, 74, respectively. The arch-shaped sectionaccording to FIG. 11 describes a 90° -bow within mounting area 46, theouter wall 18 and the inner wall 20 are connected with one another byfixture elements 22 the heads of which define an outer envelope 122 asshown in FIG. 11.

Threadings for the fixture elements 22 are manufactured in localthickenings 48 constituting a mounting area 46 of the fixture elements22, i.e. screws 42 or bolts 38. From the local thickenings 48 of theinner wall 20 of the double-walled structure 16 inwardly extending ribs36 extend into the inside area 12 of the bladder-free fuel tank 10according to the present invention. The inwardly extending ribs 36 maycomprise buckling-resistant T-shaped extensions 86 fastened to theground plate 44 of the bladder-free fuel tank 10 according to thepresent invention.

FIG. 12 finally shows a corner design of the double-walled structure ofthe bladder-free fuel tank 10 according to the present invention.

In the embodiment according to FIG. 12, a corner design 120 of thedouble-walled structure 16 is shown. The double-walled structure 16comprises the outer wall 18 and the inner wall 20, or may comprise firstand second inner wall segments 72, 74, respectively, and/or first andsecond outer wall segments 78, 80, respectively. From the interior ofthe inner wall 20 or inner wall segments 72, 74, respectively, theinwardly extending ribs 36 extend into the inside area 12, i.e. theinterior of the bladder-free fuel tank 10 according to the presentinvention. The inwardly extending ribs 36 further may comprisebuckling-resistant T-shaped or L-shaped extensions 86, respectively.

The width 27 of the hollow space 26 of the double-walled structure 16depends on the height of domes 34 present on the surface of the innerwall 20 or inner wall segments 72, 74, respectively, facing the outerwall 18. The outer wall 18 and the inner wall 20 are connected to eachother within mounting areas by fixture elements 22 such as bolts 38 orscrews 42.

As can be derived from FIG. 12, the inner wall segments 72, 74,respectively, of the inner wall 20 of the double-walled structure arefastened to each other by an inward arrangement 126 of a fixture element22. This means that the fixture element 22 connecting the inner wallsegments 72, 74, respectively, is arranged within the hollow space 26having a width 27.

Contrary, outer wall segment 78, 80, respectively, are connected witheach other by a fixture element 22 which is arranged within an outercorner recess 124. The fixture element 22 in the outer corner recess 24is arranged as not to interfere with the outer envelope 122. Which isdefined by the head of the fixture elements 22 as best shown in FIG. 12.

The double-walled structure 16, a ground plate 44 of the bladder-freefuel tank 10 according to the present invention may be made of metalmaterials such as aluminum or aluminum alloys, CFK, GFK or honeycombmaterials with respect to the outer wall 18. The space which is occupiedby the bladder-free fuel tank 10 according to the present invention isabout 6 m² to 8 m² and the usable tank capacity is in a magnitudebetween 1000 and 1500 l per minimum.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are to beincluded within the scope of the following claims.

What is claimed is:
 1. A fuel tank comprising: a reservoir formed on aninside area of the fuel tank, the reservoir being limited to an outsidearea by a double-walled structure, the double-walled structurecomprising an outer wall and an inner wall; and a rib-structurecomprising a plurality of ribs extending inwardly into the inside areafrom the double-walled structure.
 2. The fuel tank according to claim 1,wherein the outer wall is made of a material comprising aluminum and isshaped as a sheet metal part.
 3. The fuel tank according to claim 1,wherein the inner wall and/or inner wall segments are machine-milledcomponents and made from a material comprising aluminum.
 4. The fueltank according to claim 1, wherein the rib-structure extending inwardlyinto the inside area is part of the inner wall and/or inner wallsegments, respectively, or are separate components fastened to the innerwall or the inner wall segments, respectively, by at least one fixtureelement.
 5. The fuel tank according to claim 1, wherein thedouble-walled structure comprises a hollow space configured as a drainand/or venting channel for the fuel tank.
 6. The fuel tank according toclaim 1, wherein the rib structure is arranged as a cross-link structurepattern, a diagonal or triangular pattern (58), or a parallel pattern ofinwardly extending ribs.
 7. The fuel tank according to claim 1, whereinthe inwardly extending ribs comprise buckling resistant T-shaped orL-shaped extensions.
 8. The fuel tank according to claim 1, wherein theinwardly extending ribs comprise at least one local thickening and/or atleast one buckling avoiding expansion.
 9. The fuel tank according toclaim 1, wherein, within an overlapping region, edge portions of theouter wall segments overlap each other and are sealingly connected viafixtures and/or wherein, within an overlapping region, edge portions ofone inner wall segment are sealingly connected via fixtures and/or areembedded in a recess of a complementary inner wall segment.
 10. The fueltank according to claim 1, wherein the inner wall and/or the inner wallsegment, repectively, is/are milling part(s) having at least one localstrengthening portion or comprise a local thickening.
 11. The fuel tankaccording to claim 1, wherein the double-walled structure comprises onits inner wall and/or on its inner wall segments, respectively, or itsouter wall or its outer wall segments, respectively, dome-shapedelements defining a width of the hollow space of the double-walledstructure and mounting areas for the fixture elements.
 12. The fuel tankaccording to claim 1, wherein adjacent and/or opposite sections of thedouble-walled structure are interconnected by inner tie rods, the innertie rods being connected to the inwardly extending ribs at a pluralityof force transmission points.
 13. The fuel tank according to claim 12,wherein connectors or plate-shaped connectors are fastened to theinwardly extending ribs at a plurality of force transmission points andto additional ribs and/or the inner tie rod, respectively.
 14. The fueltank according to claim 1, wherein the inwardly extending ribs areformed in a height in relation to the double-walled structure and/orlarge additional riveted ribs and/or tie-rods made in a height toprevent excessive sloshing movements of fuel within the fuel tank. 15.The fuel tank according to claim 1, wherein intake equipment includingpiping, pumps, valve sensors, or slosh valves are directly mounted tothe inwardly extending ribs or to the inner wall or inner wall portions.16. The fuel tank according to claim 1, wherein the fuel tank is anadditional center tank or a center tank for an aircraft or for anaircraft application.
 17. The fuel tank according to claim 1, whereinthe fuel tank is a bladder-free tank.